The present invention generally relates to air cycle machines and, more particularly, to turbine outlet heated diffusers for air cycle machines.
A conventional air cycle machine (ACM) may be incorporated into an aircraft's environmental control system (ECS). The ACM may include a compressor operationally connected to a turbine and may compress and expand pre-conditioned engine bleed air.
In some ECS, such as those that employ a high-pressure water separation concept, the ducting just downstream of the radial turbine wheel of the ACM is susceptible to ice formation due to the extremely cold temperatures coming out of the turbine. This ice formation can be especially harmful to the ACM because it can grow back into the rotating radial turbine blades, causing the turbine blades to break and/or the turbine end journal bearings to fail because of rotor dynamic excitations as a result of the turbine wheel contacting the ice. This problem has been addressed with mixed results.
One common approach with respect to minimizing turbine icing problems is to extend the turbine inlet plenum in such a way it incorporates the diffuser structure just beyond the radial turbine outlet. In this configuration, the turbine inlet air provides the heat source to the turbine outlet section just beyond the end of the turbine wheel. But because the heating of the diffuser wall occurs with warm (and not hot) turbine inlet air it is common to put a step right at the end of the turbine wheel instead of incorporating an aerodynamic optimum turbine outlet diffuser. The step allows some ice to form, but because the turbine inlet air is greater than 32° F., the interface between the ice and the metal will get warm enough to have the ice fall off before it is thick enough to get into the turbine wheel. Unfortunately, this approach has resulted in a reduction of the turbine aerodynamic performance because of less than ideal turbine outlet geometry.
Another common approach has been to bring hot air over into a cavity at the turbine outlet area but then have this high pressure, high temperature flow exit directly into the turbine outlet. This results in a significant overall thermodynamic loss in the air conditioning system cycle because this flow had entered the cavity at high pressure and exits into the low pressure turbine outlet. This flow essentially has bypassed the turbine stage where the useful expansion of this flow is lost to the thermodynamic cycle. In addition, this special hot anti-icing flow is introduced in to the very cold turbine outlet where added heat is not desirable and results in further degradation to the air conditioning system performance.
Other solutions have included the delivery of a heated air flow to ice protecting features on the ACM in a number of different ways. Unfortunately, these solutions have required complicated plumbing schemes, which have been difficult to manufacture. Often, these protection features are not placed optimally for icing protection and often result in less than optimum turbine outlet diffusing structures.
As can be seen, there is a need for ACM ice protection that does not have a significant impact to the overall thermodynamic performance of the ECS. Specifically, ACM icing protection is needed that does not affect the ECS cycle performance by poor system design, does not result in a reduction in the turbine stage aerodynamic performance and is simple, inexpensive and easy to manufacture.